1. Field of the Invention
The present invention relates to devices for measuring periods of signals which are periodically activated, and particularly to a device for measuring a period of a horizontal synchronizing signal in a video signal and a picture signal.
2. Description of the Background Art
The display monitors of computers have conventionally included ones of multi-scanning type which make a display on the basis of video signals from a plurality of computers. Now, the periods of the horizontal synchronizing signals and the vertical synchronizing signals in the video signals are not the same among the plurality of computers. Accordingly, a setting in the display monitor must be changed corresponding to the video signals from the individual computers. More specifically, the periods of the horizontal synchronizing signals must be measured to make settings in correspondence with the periods.
The horizontal synchronizing signals may be made to be lacking in the vertical blanking intervals, particularly in the vertical synchronizing periods. Even in such a case, the periods of the horizontal synchronizing signals must be measured.
FIG. 22 is a block diagram for illustrating the structure of a conventional period measuring device 200 and FIG. 23 is a timing chart for illustrating the operation thereof. The period measuring device 200 can measure the periods of the horizontal synchronizing signals which are made to be lacking in the vertical synchronizing periods.
A horizontal synchronizing signal HS and a vertical synchronizing signal VS are applied to a microcomputer 1. The microcomputer 1 has an internal pulse oscillator 2 for generating an internal pulse IP and an internal counter 3. The internal pulse IP depends on a system clock which serves as a basis of the operation of the microcomputer 1.
The horizontal synchronizing signal HS is activated several times between adjacent activated vertical synchronizing signal VS. The period measuring device 200 measures how many times the horizontal synchronizing signal HS is activated in a certain measurement period t.sub.m.
Specifically, the measurement period t.sub.m starts when a certain waiting time t.sub.w has passed after the vertical synchronizing signal VS is activated. The measurement period t.sub.m and the waiting time t.sub.w are both set on the basis of the internal pulse IP, and for example, they are set to 1 ms and 5 ms, respectively. Then, the number of pulses of the horizontal synchronizing signal HS which are activated in the measurement period t.sub.m is counted by the internal counter 3.
Now, when the count value of pulses of the horizontal synchronizing signal HS which is measured in the measurement period t.sub.m is C, if the actual period of the horizontal synchronizing signal HS is represented by T.sub.H, then the expression (1) holds. EQU t.sub.m /(C+1)&lt;T.sub.H &lt;t.sub.m /C (1)
Accordingly, an error in the measurement of the period of the horizontal synchronizing signal HS by the period measuring device 200 is EQU .epsilon.=t.sub.m /C-t.sub.m /(C+1) (2)
As described above, the conventional art has a problem that the count value C becomes smaller as the period T.sub.H of the horizontal synchronizing signal HS becomes longer, and thus the error .epsilon. increases. Especially, when the measurement period t.sub.m is short, this problem is considerable.
For example, if the measurement period t.sub.m is 5 ms and the period T.sub.H of the horizontal synchronizing signal HS is 1/(60 kHz), the error .epsilon. is 55 ns. However, if the period T.sub.H of the horizontal synchronizing signal HS is 1/(30 kHz), the error .epsilon. becomes four times, that is, 220 ns.
Furthermore, if the measurement period t.sub.m is 1 ms, even if the period T.sub.H of the horizontal synchronizing signal HS is 1/(60 kHz), the error .epsilon. becomes 270 ns which is 4.9 times.